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Discussion Starter · #1 ·
Hello,

We have a two story house. Each story has its own system (HVAC, compressor, thermostat, etc.). In addition, the second story has a fresh-air system. However, the fresh-air system has been turned off for many years now.

Our problem is that there is air coming from outside through some vents on the first story when the AC system is completely shut down. Not all vents experience the problem and we can tell that it is air from the outside because it gets cold in winter / hot in the summer and there is a higher flow of air when it is windy outside. Also, the vents that seem to let the air go in seem to be the ones closest to the AC system inside the house. Everything works ok on the second story.

Many AC technicians have told me that it could be a loose duct in the attic that is letting air come through the vents. Therefore I had someone go up there and check the connections. No leaks or disconnections were found. Other technicians have told me that it could be the fresh-air system. However, the fresh-air system is completely off and it is connected to the second floor, not the first floor.

I can only think of two possible causes:

1) There is a duct either within the wall or within the ceiling of the first floor that is broken and unfortunately is not accessible to review. I hope that this is not the problem since I understand that it would be very hard and expensive to fix.

2) Somehow the fresh-air system may have a valve that it is broken and it is letting air coming in. Eventhough the system is off and it is connected to the second floor, maybe there is a way for air to still come in and go to the first floor.

Please can you give me your thoughts and provide any other possible causes? My family and I would really appreciate all your help.
 

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Hi Austin and welcome to the forum.
How old is the house, that may tell us if a fresh air system was required?
First, all houses need fresh air but excess air can be expensive and uncomfortable. In my energy auditing work (now retired) I would do a blower door test to measure the air exchange and follow with my infrared camera to search for those leaks. I can spot leaks buried inside walls and ceiling by the cold signature they give.

Modern technology has now made infrared cameras less expensive and often available at rental stores. Or you can purchase a home owner grade one at a low price and it will give enough information to guide you.

Once you complete the necessary air sealing a follow up blower door test will tell you if and how much fresh air is needed. No guessing.


Bud
 

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Discussion Starter · #4 ·
Thank you Bud9051 and SPS-1 work your quick replies.

To Bud9051's point, your proposal is a very good one and I will definitely consider it once I have tried other cheaper/faster alternatives. Thank you again for providing all the details.

To SPS-1's point, these are the models and manufacturers:

1st Floor: Carrier (FX4DNF049)

2nd Floor: Carrier (FX4DNF037)

The fresh-air controller on the second floor is: Aprilaire (8120A).

I am also attaching a picture of the fresh-air controller and how it is currently set.

Is there any more information that I can provide?
 

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Just a note, air leakage in the lower portions of the house tends to be infiltration which when cold is easy to detect. But an equal amount of air must leak out which occurs in the upper areas of the house and air leaking out isn't as noticeable.

The objective of the blower door test is to add sufficient exfiltration exceed the existing air leakage leaving the house which depressurizes the building resulting in all leaks becoming infiltration where the cold air will tell you where the leaks are.

It doesn't take a lot of extra fan power to force all leaks into cold incoming air and often can be achieved with a window fan and existing bath and kitchen fans.

Add in one of the low cost infrared cameras and you will become an expert leak detective and the savings will pay for that camera very quickly.

Bud
 

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Wait a minute. Re-reading your post, the fresh air system is on the second story, your problem is on the first story, and you have separate systems for the first floor and the second floor. Then the fresh air system has nothing to do with it.
 

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Discussion Starter · #8 ·
Thank you for your reply.

My hope was that the fresh-air system on the second floor was broken and somehow affecting the first floor. If that were the case, then it would be a relatively easy fix since I assume the fresh-air system is relatively simple to diagnose.

SPS-1, if I follow your comments on not focusing on the fresh-air system because there is no way that it would affect the first floor, what next step do you recommend? Should I contact a professional to come and check the system? If so, should I ask for an estimate on using the camera to look inside the duct in order to compare that dollar amount vs using the home-grade cameras that Bud was suggesting?

Also, could you please let me know any other test/solution that I could try on my own before calling a professional? I am hoping on saving some money if possible.

Thank you for all your help in advance.
 

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Discussion Starter · #9 ·
I forgot to mention.

Some technicians say that it could also be a pressure differential between the inside and outside of the house. I don't think this is the case because the problem appeared out of nowhere around 2 years ago. Our house is 5 years old and for the first 3 years we didn't have any problem at all.

Finally, I must also say that at some point I changed the default thermostat to a couple of Ecobees. I don't think this is the issue either since the problem happens when the system is off, but I thought sharing the information was beneficial.

Thanks again!
 

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Discussion Starter · #11 ·
Hi guys,

Thank you for your replies.

This is an update. I had two technicians come here. They could not find an explanation for the air. They said that the draft was too weak for it to be caused by leaks on the ducts. They also said that the system is working just fine when the heat is on. If there were a leak on the duct, according to them the heat would not come through either.

Another possibility is that the air is circulating from the return, passing through the ducts in the attic, and coming down the vents. When passing through the ducts, the air is getting cooler but not because there is a leak but because the attic is just very cold and it is impacting the air inside the duct. What do you think about this possible cause? Does it make sense to you?

Thanks again for all the help.
 

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Discussion Starter · #13 ·
Hi SeniorSitizen,

Yes, the technicians also double-checked the fresh air system on the second floor and were able to confirm that it is working properly and that it is completely independent of the first floor, which is where we have the problem. So, even though the technicians did not find a solution, at least they ruled out the fresh air system.
 

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Hi SeniorSitizen,

Yes, the technicians also double-checked the fresh air system on the second floor and were able to confirm that it is working properly and that it is completely independent of the first floor, which is where we have the problem. So, even though the technicians did not find a solution, at least they ruled out the fresh air system.
I'm saying take that system completely out, pitch it in a dumpster and plugging / sealing all openings that it was concerned with.
 

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The technician's hypothesis is a possibility. That would just be natural convection. Hot air rises, cools off in the attic, then falls.

Any HVAC guys here seen that before?

But then you should be finding some vents with air flowing in, and some vents with air flowing out. And in a few months, when in 75 outside, you should see no air flow.

I remember I used to work in a room that used to be a showroom for a manufacturer. Glass windows all the way to the ceiling, which must have been 16 ft high. In the winter, hot air would rise to the ceiling, hit the windows, cool off and then fall, then hit the table behind me and the air would roll off the table. It was like a wind. If you put a hot cup of coffee on the table, you could see the steam moving horizontally.
 

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There is the possibility that the air you are detecting is coming from inside the wall cavity and not from inside the vent. Any wall penetration is subject to air leakage from infiltration from numerous sources. We see it often at a thermostat where it causes unusual effects on response.

Perhaps a careful test with a smoke candle would tell something interesting.


Following that same line of thought, if the air infiltrating comes into contact with the metal ducting, it could cool the duct down to the point where the air inside the duct would also get cool and tend to flow downward as cold air does from gravity. The value of the duct insulation would be a consideration.
 

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Hi guys,

Thank you for your replies.

This is an update. I had two technicians come here. They could not find an explanation for the air. They said that the draft was too weak for it to be caused by leaks on the ducts. They also said that the system is working just fine when the heat is on. If there were a leak on the duct, according to them the heat would not come through either.

Another possibility is that the air is circulating from the return, passing through the ducts in the attic, and coming down the vents. When passing through the ducts, the air is getting cooler but not because there is a leak but because the attic is just very cold and it is impacting the air inside the duct. What do you think about this possible cause? Does it make sense to you?

Thanks again for all the help.
That's not an uncommon occurrence in systems with their duct work running through the attic.
 
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